Quantum communication has presented a revolutionary step in secure communication due to its high security of the quantum information, and many models including quantum key distribution, quantum teleportation and quantum secure direct communication (QSDC) have been developed. Quantum secure direct communication based on entanglement can directly transmit confidential information. QSDC sends secret information directly over a secure quantum channel. Any attack of QSDC results to only random number, and cannot obtain any useful information from it. Therefore, QSDC has simple communication steps and reduces potential security loopholes, and offers high security guarantees, which continues to enhance the security and the value propositions of quantum communications in general.

Recently, the experimental QSDC has been developed significantly. However, the inability to simultaneously distinguish the four sets of encoded orthogonal entangled states in entanglement-based QSDC protocols limits its practical application. Furthermore, it is important to construct quantum network in order to make wide applications of quantum secure direct communication. Experimental demonstration of QSDC is badly required.

In the present study, we explored a QSDC network based on time-energy entanglement and sum-frequency generation as shown in Figure 1. Using the frequency correlations of the fifteen photon pairs via time division multiplexing and dense wavelength division multiplexing, we perform a 40-kilometer fiber QSDC experiment by implying two-step transmission between each user as shown in Figure 2. 15 users are in a fully connected QSDC network, and the fidelity of the entangled state shared by any two users is greater than 97%.

Figure 1 Composition of quantum network. (a) Communication network (b) Subnet

The results show that when any two users are performing QSDC over 40 kilometers, the fidelity of the entangled state shared by them is still greater than 95%, and the rate of information transmission can be maintained above 1Kbp/s. Our result demonstrates the feasibility of a proposed QSDC network, and hence lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.

Figure 2 Experimental set-up. (a) The physical structure of the quantum network. (b) The spectrum. (c) Illustration of SFG progress.

This research was published in “Zhantong Qi, Yuanhua Li, Yiwen Huang, Juan Feng, Yuanlin Zheng, and Xianfeng Chen, A 15-user quantum secure direct communication network, Light: Science & Applications, 10, 183 (2021)”.

Link: https://doi.org/10.1038/s41377-021-00634-2